Hard disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, user data are magnetically stored on one or more discs that are rotated at a constant high speed. A rotary actuator supports a plurality of read/write heads that fly adjacent the surfaces of the discs to write data to and read data from tracks defined on the discs.
A voice coil motor (VCM) is used to rotate the actuator, with a typical VCM comprising a magnetic circuit which uses an array of permanent magnets and magnetically permeable pole pieces to generate a magnetic field. A coil of the actuator assembly is immersed in this magnetic field. Application of current to the coil establishes a second magnetic field which interacts with the magnetic field of the magnetic circuit to cause the coil to move laterally, pivoting the actuator about a pivot shaft adjacent the discs. As the actuator pivots, the heads are moved across the disc surfaces. Position information is detected from servo data written to the tracks and used by a servo circuit to control the bidirectional application of current to the coil. A typical processor based digital servo system is described by U.S. Pat. No. 5,262,907, assigned to the assignee of the present invention.
When the disc drive is deactivated, the rotating discs are brought to a stop and the heads are moved to a parked position, such as a texturized landing zone near the innermost diameters of the discs, and the actuator is latched to prevent inadvertent movement of the heads out onto the disc surfaces. While the heads are advantageously provided with aerodynamic features which enable the heads to fly in very close proximity to the rotating discs, such features will induce significant adhesive forces ("stiction") against the smooth discs should the heads come to rest on the discs when the discs are not rotating. Such stiction can be sufficiently great to prevent the discs from starting again, so movement of the heads out onto the nonrotating disc surfaces can result in a catastrophic failure of the disc drive.
Accordingly, disc drive manufacturers have attempted to provide ever greater levels of disc drive non-operating mechanical shock resistance, including the implementation of ramps at the outermost diameters of the discs which lift the heads up off of the disc surfaces when the disc drive is deactivated. Since the heads are aerodynamically supported above the rotating discs, the leading edges of the ramps extend out over at least a portion of the outermost diameters of the discs to allow the actuator to sweep the heads over to and up the ramps to the final parked position, which is typically beyond the outermost diameters of the discs. Thus, use of ramp loading typically requires an actuator to operate over a greater angular range ("sweep angle") than that merely required to access all of the tracks on the disc surfaces.
Increasing the sweep angle of the actuator can be accomplished by increasing the dimensions of the magnetic circuit or the coil, but such design modifications typically result in greater costs due to the extra magnet, steel and wire material required. Also, increasing the mass of the coil affects the size and balance of the actuator, which can adversely impact operational performance of the actuator.
There is a need, therefore, for a way to increase the operational sweep angle of an actuator without using modifications that add size and decrease operational performance of the actuator. It is to such improvements that the present invention is directed.